Logging rides is not a necessity for me, but I like it. I log my rides in a program called CycliStats. This is a nice tool for keeping track of lots of things related to my riding. Here are the kinds of things it tracks:

Outdoor rides

Indoor rides

Maintenance intervals

Goal attainment

If you're looking for a tracking tool for riding, I haven't found anything better. It takes less than a minute to load the program and enter a ride. Well worth it to me for what I get out of it:

A quick visual display of when I last took a day off.

Weekly and monthly ride totals (distance, time, calories, climbing)

Cumulative data graphs (total climbing is my favorite)

Adding a ride is simple, and very little is actually required. Here's the data I enter for each ride:

Time the ride started (date is selected automatically when I right click in the calendar

Which bike I rode (road bike or spin bike)

Distance (for frequent rides this is entered automatically when I choose the pre-define route)

Time (I use the moving time data from my GPS via TopoFusion)

Elevation gain (I get that from my GPS via TopoFusion)

Heart rate, average and max (from my heart rate monitor)

There are a lot of other data entry fields, but these work for me. If you're interested in logging your rides, you should check out CycliStats.